In general, a GNSS system is made up of a plurality of satellites, or constellation of satellites, allowing a portable geolocation receiver to determine positioning information, in a land-based plane of reference, also called position, speed and time (PST) information.
There are currently several GNSS systems, which in particular include the GPS system, the GLONASS system or the GALILEO system, which is expected to be brought online soon.
The satellites of such a GNSS system are able to emit radio signals in particular comprising navigation information.
Each item of navigation information generally comprises data relative to the transmission time by the satellite of the corresponding signal and the current position of the satellite. In particular, the data relative to the current position of the satellite generally contain the almanac providing a rough position of the satellite and the ephemerides giving the exact current position of the satellite.
The item of navigation information is carried by a carrier wave and modulated by a spreading code specific to each satellite. Thus, the signals are emitted by the satellites using a spread spectrum technique.
The geolocation receiver, also called GNSS receiver, is able to receive the signal emitted by the satellites and to extract the navigation information therefrom in particular to determine the distance to the transmitting satellite that transmitted the corresponding signal.
This distance, also called pseudo-distance, is determined by analyzing the propagation delay of the corresponding signal.
To determine the PST positioning information, the receiver performs a digital processing of the navigation information from at least three different satellites.
In practice, to have a more precise position, the receiver needs navigation information from at least four different satellites.
More specifically, to acquire the navigation information from a given satellite, the receiver implements two phases processing the signals from this satellite.
During an initial phase, called acquisition phase in the state of the art, the receiver generates a local signal in particular containing a local spreading code showing the image of the spreading code of the satellite.
Since the receiver does not initially know its position, the local signal is not synchronized with the received signal. This in particular means that the local signal is carrier frequency-shifted from the received signal by a value called Doppler value, and the spreading code of the received signal is delayed from the local spreading code by a value called delay value.
The receiver then searches for a peak in the correlations between the local signal and the received signal by trying different Doppler and delay values.
When a peak is detected, the receiver determines the Doppler and delay values corresponding to this peak and from these values, launches a following phase, called continuation phase in the state of the art.
During the continuation phase, the receiver regularly updates the Doppler and delay values, and extracts the item of navigation information from the signal emitted by the satellite in particular using the local spreading code and the determined Doppler and delay values.
At the end of the acquisition phase, the receiver is considered to have synchronized itself with the emitting satellite or has “locks on” to this satellite, owing to the detection of the correlation peak.
Sometimes, the receiver synchronizes its local signal corresponding to the desired satellite with the signal received from another satellite, which leads to an erroneous distance measurement, and therefore potentially a false position.
In this case, this is a false synchronization or a false “lock”, which is also called cross-correlation. In this case, the computation of the position information of the receiver is distorted.
In particular, the cross-correlation error occurs when the satellites emit GNSS signals with a short periodic code, for example GPS L1 C/A (“coarse acquisition”) signals, corresponding to a frequency of 1575.42 MHz, SBAS L1 C/A and GALILEO L1 BC.
A similar phenomenon also occurs when the received signal comes from a satellite transmitting position correction information, in a space-based augmentation system, called SBAS system.
Different methods exist in the state of the art making it possible to avoid such false synchronization or cross-correlation.
Thus, one method, used conventionally, consists of verifying the consistency between the position of the satellite computed from ephemerides contained in the item of navigation information and that computed from the decoded almanacs. The almanacs contain the identifiers of all of the transmitting satellites in the constellation, unlike the ephemerides. An inconsistency between these values therefore indicates a false synchronization.
Indeed, the ephemeris data from the satellite makes it possible to estimate the position of this satellite with a precision of several meters, but is only transmitted by the satellite itself and is only valid for several hours.
The almanac data for the entire constellation of satellites makes it possible to estimate the position of each of the satellites roughly, to within several hundred kilometers, but is transmitted by all of the satellites in the constellation and is valid for several days.
Thus, if the difference between the position computed from the ephemerides and the position computed from the almanacs is greater than the mean distance between transmitting satellites, an error, and therefore a cross-correlation, is considered to exist.
In order to avoid a false cross-correlation detection and guarantee the integrity of the positioning information computed by a GNSS receiver, RTCA (Radio Technical Commission for Aeronautics) standard DO-229 “Minimum Operational Performance Standards for Global Positioning System” requires complete decoding of all of the received ephemeris data twice and a comparison with the decoded almanac data for all of the satellites.
For a given transmitting satellite, a complete set of ephemeris data is made up of a given number of ephemeris words, each having an associated rank and encoding information relative to the transmitting satellite, this information making it possible to compute the position of the transmitting satellite. The ephemeris data is resent periodically, and refreshed at a given frequency, for example every two hours for GPS satellites.
The ephemeris words are sent in sub-frames of the transmitted signal, the transmission of all of the words of an ephemeris requiring a plurality of sub-frames.
Thus, the traditional method for validating the absence of a cross-correlation takes a relatively long amount of time, which is from 48 seconds to 60 seconds for the GPS system and several minutes for the SBAS system.
The present invention aims to resolve this drawback.